In the operation of a converter, there are two methods for controlling the same, i.e. static controlling and dynamic controlling.
In the static controlling, the accuracy of the end point is extremely bad, while in the dynamic controlling, the accuracy of the end point (percentage of obtaining proper values of carbon and temperature) is about 100%, and under such conditions of operation, molten steel is poured out within 30 to 60 seconds after shutdown of the oxygen blowing for refining.
In the usual static controlling, the pouring out of molten steel is effected, using about 4 to 10 minutes, after the shutdown of the oxygen blowing, for temperature measurement sampling, re-refining, etc. However, recently the pouring out of the molten steel has been effected for 30 to 60 seconds after the shutdown of oxygen flowing to improve the operation of the converter as well as the productivity of the steel, although a significant problem has arisen in the operation of steel making.
Namely, immediately after oxygen blowing for refining, formed slag and oxygen in molten steel are not under an equilibrium state, so that if the molten steel is poured out immediately after shutdown of the oxygen blowing, the slag and the molten steel will react for 3 to 7 minutes during the pouring out of the molten steel to cause a decarburization reaction, thus decreasing the carbon in the molten steel. It has been found that the temperature will lower due to boiling by the reaction at this time. This means that although the blowing values of the carbon and temperature at the time of the shutdown of the oxygen are proper, if the molten steel is poured out immediately, the composition and temperature thereof within a ladle will not meet our expectations due to the reaction between the slag and molten steel during the pouring out thereof. This is a very important problem on the quality control.
As shown in FIG. 3, the reaction between slag and molten steel continues for at least 3 to 4 minutes after shutdown of the oxygen blowing, and even if the carbon and temperature are proper in the dynamic control, unless the molten steel is kept stationary for 3 to 4 minutes and is poured out after completion of the reaction between slag and molten steel, the object of the operation of steel making and the effects of the dynamic control on the steel making are significantly decreased or lost altogether.